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Water Molecule Water’s properties can best be understood by considering the structure and bonding of the water molecule: The water molecule is made up of two hydrogen atoms bonded to an oxygen atom. The three atoms are not in a straight line; instead, as shown above, they form an angle of 105°. Because of water’s bent structure and the fact that the oxygen atom attracts the negative electrons more strongly than do the hydrogen atoms, the water molecule behaves like a dipole having opposite electrical charges at either end.

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The properties of water are due to the polar nature of the water molecule and its ability to form hydrogen bonds. The Water Molecule 04/12/2012Kimia Lingkungan Air-18

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Sifat Panas Air yang Penting High heat capacity of 4.184 joules per gram per ˚ C (J/g- ˚C) Very high heat of fusion of 334 joules per gram (J/g) Very high heat of vaporization of water is 2,259 J/g, water vapor carries latent heat 04/12/2012Kimia Lingkungan Air-121

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conserve water Since about 1980, however, water use in the U.S. has shown an encouraging trend with total consumption down by about 9% during a time in which population grew 16%, according to figures compiled by the U.S. Geological Survey. This trend, which is illustrated in Figure 3.2,

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conserve water It has been attributed to the success of efforts to conserve water, especially in the industrial (including power generation) and agricultural sectors. Conservation and recycling have accounted for much of the decreased use in the industrial sector. Irrigation water has been used much more efficiently by replacing spray irrigators, which lose large quantities of water to the action of wind and to evaporation, with irrigation systems that apply water directly to soil. Trickle irrigation systems that apply just the amount of water needed directly to plant roots are especially efficient

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Distribution Problem of Water A major problem with water supply is its nonuniform distribution with location and time. As shown in Figure 3.3, precipitation falls unevenly in the continental U.S. This causes difficulties because people in areas with low precipitation often consume more water than people in regions with more rainfall. Rapid population growth in the more arid southwestern states of the U.S. during the last four decades has further aggravated the problem

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THE CHARACTERISTICS OF BODIES OF WATER The physical condition of a body of water strongly influences the chemical and biological processes that occur in water. Surface water occurs primarily in streams, lakes, and reservoirs. Wetlands are flooded areas in which the water is shallow enough to enable growth of bottom-rooted plants. Estuaries are arms of the ocean into which streams flow. The mixing of fresh and salt water gives estuaries unique chemical and biological properties. Estuaries are the breeding grounds of much marine life, which makes their preservation very important

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Water’s unique temperature-density relationship results in the formation of distinct layers within nonflowing bodies of water, as shown in Figure 3.6. During the summer a surface layer (epilimnion) is heated by solar radiation and, because of its lower density, floats upon the bottom layer, or hypolimnion. This phenomenon is called thermal stratification.

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THE CHARACTERISTICS OF BODIES OF WATER When an appreciable temperature difference exists between the two layers, they do not mix but behave independently and have very different chemical and biological properties. The epilimnion, which is exposed to light, may have a heavy growth of algae. As a result of exposure to the atmosphere and (during daylight hours) because of the photosynthetic activity of algae, the epilimnion contains relatively higher levels of dissolved oxygen and generally is aerobic. In the hypolimnion, bacterial action on biodegradable organic material may cause the water to become anaerobic (lacking dissolved oxygen). As a consequence, chemical species in a relatively reduced form tend to predominate in the hypolimnion.

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THE CHARACTERISTICS OF BODIES OF WATER The shear-plane, or layer between epilimnion and hypolimnion, is called the thermocline During the autumn, when the epilimnion cools, a point is reached at which the temperatures of the epilimnion and hypolimnion are equal. This disappearance of thermal stratification causes the entire body of water to behave as a hydrological unit, and the resultant mixing is known as overturn. An overturn also generally occurs in the spring. During the overturn, the chemical and physical characteristics of the body of water become much more uniform, and a number of chemical, physical, and biological changes may result. Biological activity may increase from the mixing of nutrients. Changes in water composition during overturn may cause disruption in water-treatment processes.

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Sources and Uses of Water: The Hydrologic Cycle The world’s water supply is found in the five parts of the hydrologic cycle(Figure 3.1). About 97% of Earth’s water is found in the oceans. Another fraction is present as water vapor in the atmosphere (clouds). Some water is contained in the solid state as ice and snow in snowpacks, glaciers, and the polar ice caps. Surface water is found in lakes, streams, and reservoirs. Groundwater is located in aquifers underground.

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The Hydrologic Cycle: There is a strong connection between the hydrosphere, where water is found, and the lithosphere, which is that part of the geosphere accessible to water. Human activities affect both. For example, disturbance of land by conversion of grasslands or forests to agricultural land or intensification of agricultural production may reducevegetation cover, decreasing transpiration (loss of water vapor by plants) and affecting the microclimate. The result is increased rain runoff, erosion, and accumulation of silt in bodies of water. The nutrient cycles may be accelerated, leading to nutrient enrichment of surface waters. This, in turn, can profoundly affect the chemical and biological characteristics of bodies of water.

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Where Earth’s Water is Found  About 97% of Earth ’ s water is in oceans  Most of the remaining water is in the form of solid snow and ice  Less than 1% of Earth ’ s water as water vapor and clouds in the atmosphere, as surface water in lakes, streams, and reservoirs, and as groundwater in underground aquifers

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7.4. Bodies of Water and Life in Water Stratification of a Body of Water Strongly Affects Chemical and Biological Processes

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CHEMICAL PROCESSES IN WATER Many chemical and biochemical reactions occur in water in the environment. Theseare discussed here on the basis of their chemical classification. Several of these were shown by example reactions in Figure 7.4 1. The photosynthesis reaction, which utilizes sunlight energy to produce biomass, 04/12/2012Kimia Lingkungan Air-151

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CHEMICAL PROCESSES IN WATER is shown here for the conversion of inorganic carbon from dissolved HCO3- ion to organic carbon (biomass) abbreviated as {CHO}. This reaction produces biomass that can be acted upon biochemically by other organisms to form the basis of a number of important biochemical processes in water 04/12/2012Kimia Lingkungan Air-152

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CHEMICAL PROCESSES IN WATER 2. The carbonate ion, CO3 2- generated by photo- synthesis reacts with water removing a hydrogen ion, H, from the water molecule and producing OH - ion. Reactions involving the exchange of H+ or the generation or consumption of OH- are acid-base reactions. This reaction generates OH- ion, so it makes the water more basic. The carbonate ion generated by photosynthesis may become involved in another kind of reaction as exemplified by its reaction with dissolved calcium ion, Ca 2+, in water, to produce solid CaCO3 04/12/2012Kimia Lingkungan Air-153

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CHEMICAL PROCESSES IN WATER This is a precipitation reaction. CaCO3 is limestone, and it is this kind of reaction, beginning with the CO3 2- generated by photosynthesis, that is responsible for large formations of limestone rock throughout the world. 3. Oxidation-reduction reactions (see Section 4.7), usually carried out by bacteria, are common in natural waters. The bacterially- mediated reaction of sulfate ion, SO4 2- acting as an oxidizing agent in the O 2 - deficient bottom regions of a body of water to oxidize biodegradable organic matter, {CH 2 O} 04/12/2012Kimia Lingkungan Air-154

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CHEMICAL PROCESSES IN WATER Reaksi : is one in which the sulfate ion loses oxygen (is reduced). As the H 2 S gas bubbles up through the water, it may contact molecular oxygen and other kinds of bacteria that cause it to undergo the following reaction in which the sulfur is oxidized with the addition of oxygen atoms to produce SO4 2 -ion: 04/12/2012Kimia Lingkungan Air-155

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7.6. FIZZY WATER FROM UNDERGROUND  Natural waters contain dissolved gases.  Dissolved oxygen required by fish  Dissolved carbon dioxide in some mineral waters  Carbon dioxide in Lake Nyos in the African country of Cameroon which asphyxiated 1,700 people in 1986 Henry ’ s Law for gas solubilities states that the solubility of a gas in a liquid is proportional to the partial pressure of that gas in contact with the liquid. Gas solubility decreases with increasing temperature

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7.7. (WEAK) ACID FROM THE SKY  An acid is a substance that contains or produces H + ion in water, whereas a base is a substance that accepts H + ion in water or contains or produces hydroxide ion, OH -  Whether water is acidic or basic is expressed by pH:  pH = -log [H + ] (7.7.1)  [H + ] is the molar concentration of H + in water, that is, the number of moles of this ion per liter of water. [H + ], mol/Llog[H + ]pH 0.100-1.001.00 1.00  10 -3 -3.003.00 1.00  10 -5 -5.005.00 1.00  10 -9 -9.009.00

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Acid in Water (Continued) The value of [H + ] in pure water at 25˚ C is 1.00  10 -7 mol/L and the pH is 7.00. The concentration of dissolved carbon dioxide, [CO 2 (aq)], in water in equilibrium with 370 ppm atmospheric air at 25˚ C is 1.21  10 -5 mol/L. Makes water slightly acidic because CO 2 + H 2 O  H + + HCO 3 - (7.7.2) [H + ] = 2.3  10 -6 mol/L corresponding to a slightly acidic pH of 5.6 Such water is neutral, neither acidic nor basic. Water with a pH less than 7.00 is acidic, whereas water with a pH greater than 7.00 is basic. The average global concentration of CO 2 gas in air in the year 2001 was about 370 parts per million by volume, and going up by about 1 ppm per year.

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Atmospheric CO 2 dissolved in water, and from biodegradation HCO 3 - dissolved in water Solid carbonates (CaCO 3 ) in mineral formations in contact with water Carbon Dioxide and Carbonate Species in Water

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7.9. METALS IN WATER Metal ions in water are present as hydrated ions, such as Ca(H 2 O) 6 2+. Bound water molecules can be displaced reversibly by other species. Such species include chelating agents, which can bond to metal ions in 2 or more places to form a metal chelate. One such chelating agent is the nitrilotriacetate anion used in some cleaning formulations and capable of bonding to a metal ion on 4 separate sites Chelates tend to be particularly stable, and they are very important in natural water systems. Chelates are involved in life systems; for example, blood hemoglobin is a chelate that contains Fe 2+ ion bonded simultaneously to 4 N atoms on the hemoglobin protein molecule

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Humic Substances in Water  Water in nature may contain naturally-occurring chelating agents called humic substances that are complex molecules of variable composition left over from the biodegradation of plant material.  Humic substances bind with Fe 2+ ion to produce gelbstoffe (German for “ yellow stuff ” ) which is very difficult to remove by water treatment processes.  Humic substances produce trihalomethanes, such as chloroform, HCCl 3 during disinfection of water by chlorine

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Most important chemical and biochemical processes in water occur at interfaces between water and another phase (usually solid) 7.10. Water Interactions With Other Phases

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 Very small particles suspended in water  Size ranging from very large molecules up to about 1  m  Scatter light (Tyndall effect)  Unique characteristics High surface/volume High interfacial energy High surface/charge Colloids in Water

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Behavior and stability of colloids are important in aquatic chemical phenomena  Formation of sediments  Dispersion and agglomeration of bacterial cells  Dispersion and removal of pollutants  Waste treatment processes

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AQUATIC LIFE The living organisms (biota) in an aquatic ecosystem may be classified as either autotrophic or heterotrophic. Autotrophic organisms utilize solar or chemical energy to fix elements from simple, nonliving inorganic material into complex life molecules that compose living organisms. Algae are the most important autotrophic aquatic organisms because they are producers that utilize solar energy to generate biomass from CO2 and other simple inorganic species.

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AQUATIC LIFE Heterotrophic organisms utilize the organic substances produced by autotrophic organisms as energy sources and as the raw materials for the synthesis of their own biomass. Decomposers (or reducers) are a subclass of the heterotrophic organisms and consist of chiefly bacteria and fungi, which ultimately break down material of biological origin to the simple compounds originally fixed by the autotrophic organisms.

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AQUATIC LIFE The ability of a body of water to produce living material is known as its productivity. Productivity results from a combination of physical and chemical factors. High productivity requires an adequate supply of carbon (CO2), nitrogen (nitrate), phosphorus (orthophosphate), and trace elements such as iron Water of low productivity generally is desirable for water supply or for swimming.

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AQUATIC LIFE Relatively high productivity is required for the support of fish and to serve as the basis of the food chain in an aquatic ecosystem. Excessive productivity results in decay of the biomass produced, consumption of dissolved oxygen, and odor production, a condition called eutrophication

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AQUATIC LIFE Life forms higher than algae and bacteria—fish, for example—comprise a comparatively small fraction of the biomass in most aquatic systems. The influence of these higher life forms upon aquatic chemistry is minimal. However, aquatic life is strongly influenced by the physical and chemical properties of the body of water in which it lives. Temperature, transparency, and turbulence are the three main physical properties affecting aquatic life. Very low water temperatures result in very slow biological processes, whereas very high temperatures are fatal to most organisms. The transparency of water is particularly important in determining the growth of algae. Turbulence is an important factor in mixing processes and transport of nutrients and waste products in water. Some small organisms (plankton ) depend upon water currents for their own mobility.

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Dissolved oxygen (DO Dissolved oxygen (DO) frequently is the key substance in determining the extent and kinds of life in a body of water. Oxygen deficiency is fatal to many aquatic animals such as fish. The presence of oxygen can be equally fatal to many kinds of anaerobic bacteria. Biochemical oxygen demand, BOD, discussed as a water pollutant, refers to the amount of oxygen utilized when the organic matter in a given volume of water is degraded biologically.

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Carbon dioxide Carbon dioxide is produced by respiratory processes in water and sediments and can also enter water from the atmosphere. Carbon dioxide is required for the photosynthetic production of biomass by algae and in some cases is a limiting factor. High levels of carbon dioxide produced by the degradation of organic matter in water can cause excessive algal growth and productivity. The salinity of water also determines the kinds of life forms present. Irrigation waters may pick up harmful levels of salt. Marine life obviously requires or tolerates salt water, whereas many freshwater organisms are intolerant of salt.